Current theory is that 2019 Corona Virus originated in bats.

Bats themselves did not get sick from it, but they passed it to humans through an intermediary host, another animal that did get sick, and eventually ended up in contact with their first human.

Viruses are mutating all the time, and the scientists are able to use these mutations to estimate the duration of mutations. Based on this evidence, scientists believe this virus has only been in humans since around since mid November, early December 2019.